Epitaxial layers are often provided on silicon wafer substrates highly doped with P-type or N-type dopants in order to improve performance of semiconductor devices.
When dopant atoms are highly doped on silicon substrates, difference in the size (covalent radius) of the dopant atoms and silicon substrate atoms results in a strain in crystal to generate an internal stress. The magnitude of the generated stress increases in accordance with the thickness of the epitaxial layers.
When a lowly doped epitaxial layer is provided on such a highly doped semiconductor substrate, misfit dislocation occurs at an interface between the substrate and the epitaxial layer, which is transmitted to the topmost surface of the epitaxial layer. The generated dislocation causes junction leakage and the like to impair the performance of the semiconductor devices.
It has therefore been proposed that, when boron (B) is used as P-type dopant and to be highly doped, germanium (Ge) is doped together with boron (see, for instance, Patent Documents 1 and 2).
According to Patent Document 1, the added germanium restrains decrease in lattice constant of silicon caused on account of the large amount of added boron, so that generation of the misfit dislocation can be effectively restrained.
In the above Patent Document 2, since boron and germanium are added as dopants, the lattice constant that is changed under the influence of the highly doped boron at a site near an interface with a thermally-oxidized film of silicon wafer is compensated by germanium. Consequently, no strain occurs on a wafer facial layer after the thermally oxidized film is removed and the wafer is not deformed while growing the epitaxial layer, so that no misfit dislocation occurs in the epitaxial layer.
On the other hand, Patent Document 3 discloses an exemplary use of phosphorus (P) as an N-type dopant. According to the technique disclosed in Patent Document 3, germanium is added when a large amount of phosphorus is doped in silicon to compensate for the strain in crystal lattice caused by phosphorus to prevent the misfit dislocation from being generated.
[Patent Document 1] JP-A-2004-175658
[Patent Document 2] JP-A-2005-223092
[Patent Document 3] JP-A-09-7961